The design principles and fabrication of conventional MOS transistors are well known. Typically, an MOS transistor includes a channel region in a semiconductor substrate between a doped source region and a doped drain region in the substrate. A conductive gate electrode overlies the channel region, physically separated from the substrate by a thin silicon oxide insulating layer, termed the gate oxide.
MOS transistor performance is characterized by a drive current, which is the current flowing between source and drain regions when the transistor is conducting, and by circuit delay. For desirable performance, the goal is to maximize the drive current and to minimize circuit propagation delay for a given geometry.
Polysilicon is frequently used as the material for forming the gate electrode for MOS transistor devices. Polysilicon adheres well to gate oxides and is able to withstand the environmental changes during processing steps subsequent to the gate electrode being formed. An impurity species, or dopant, is incorporated in the polysilicon to make the polysilicon conducting. The dopant species used in conventional transistor processing include phosphorous and arsenic, for creating n-type polysilicon, and boron or BF.sub.2, for creating p-type polysilicon. Conventionally, a high dopant concentration is used to reduce the degree of depletion in the polysilicon gate. The degree of depletion is the thickness of the region in the polysilicon gate adjacent the gate oxide layer, in which space charge develops when the transistor device is conducting. The conventional approach is to minimize the level of depletion, which may result in an improvement in drive current.
However, maximizing the dopant concentration to minimize the level of depletion can have additional effects. The higher the doping level, the more defects are created in the polysilicon. When boron is used as a dopant, higher concentrations of boron increase the likelihood of boron penetration, the tendency of boron to diffuse out of the polysilicon gate into the gate oxide layer, which undesirably can affect device reliability. Furthermore, minimizing the level of depletion does not necessarily improve propagation delay. Under certain conditions, as depletion levels are decreased, circuit delay can actually increase.
Thus, it would be desirable to provide a design for an MOS transistor that simultaneously increases drive current and device speed, and minimizes circuit delay. It would further be desirable to provide a method of manufacturing such a device.